The invention relates to an optic device for detection of characteristics of particles in movement in a fluid flowing along a wall, a device comprising a light source, means, connected to the light source, for producing at least two mutually coherent light beams forming between them a predetermined angle so as to form an interference zone constituting a measuring volume on the path of the fluid, optic means for detection of the light diffused by particles passing through the measuring volume, optoelectronic means for detection, connected to the optic means for detection and supplying electronic signals representative of the light diffused, and processing means, connected to the optoelectronic means for detection.
A Doppler effect laser velocimeter enables the velocity of submicronic particles in a fluid to be measured. A velocimeter of this kind is based on particles passing through a succession of equidistant dark and bright zones belonging to a network of interference fringes.
FIG. 1 illustrates the principle of Doppler effect laser velocity measurement using the fringes method. Two coherent light beams 1 and 2, originating from the same laser 3, cross at the focal point of a convergent lens 4 inside a measuring volume 5. FIG. 2 represents the measuring volume 5 in greater detail. Inside the measuring volume 5, a system of interference fringes appears disposed according to equidistant planes parallel to the internal bisecting line of the two beams 1 and 2. The distance between two successive planes is the interfringe distance i. A particle driven by a fluid in movement perpendicularly to the fringes (axis X'X of FIG. 2, arrow 6 of FIG. 1), crosses the measuring volume 5 and passes successively through in the dark and bright fringes. The light flux diffused by a particle is modulated at a frequency proportional to its velocity. The light diffused by a particle is collected by an optic detection device. The diffused light is generally measured by retro-diffusion, that is to say on the side where the emission source constituted by the laser 3 is located, as represented in FIG. 1. The detection device comprises an objective 7 with fairly wide opening to collect the light diffused by the particles and form the image of the measuring volume on a diaphragm situated in front of a photo-multiplier 8. The latter supplies electrical signals representative of the light diffused by the particles to an electronic processing circuit 9. The signals supplied to the processing circuit 9 are presented in the form of a succession of sine wave trains, as represented in FIG. 3. Each of the trains, one of which is represented between two broken lines, is due to the passage of a particle in the measuring volume 5. The frequency of a sine wave train is representative of the velocity of the corresponding particle in the measuring volume. The processing circuit performs a frequency analysis of the signals. Known processing techniques use spectral analysis, frequency followers or image correlation. These various techniques enable mean values of the velocity of a particle to be obtained. Counting techniques, for their part, enable instantaneous values of the velocity of a particle to be obtained.
The Doppler effect laser velocimeter has up to now remained reserved for laboratory measurements. Known velocimeters are costly and require difficult adjustments, notably adjustment of the angle A (FIG. 1) of the beams 1 and 2. Modification of the angle A in fact modifies the interfringe distance i, and consequently the frequency of the measured signals. The optic detection device also has to be carefully aligned.
The Doppler effect laser velocimeter has up to now been used more particularly for measuring the velocity of particles in a fluid or, indirectly, for friction measurement near a wall. With known velocimeters, a section of the wall has to be transparent. The light beams reach the wall with a low incidence and the signals collected are very noisy on account of the light reflections on the walls. Only long and delicate optic adjustments, impracticable in the industrial world, enable acceptable signal-to-noise ratios to be obtained, in a laboratory.